1. Field of the Invention
The present invention relates to an oscillation circuit using a piezoelectric oscillator and electronic equipment comprising a semiconductor integrated device with a clock function including the oscillation circuit.
2. Description of the Related Art
There are many conventional electronic equipment, such as digital cameras, that have a clock function to display or record time in real-time. In order to implement the clock function, it is common to comprise an oscillation circuit for a clock and count time by the constant oscillation of the oscillation circuit even if the electronic equipment isn't used. Therefore if the electronic equipment is for portable use, low power consumption is demanded for the oscillation circuit so that the battery to supply power does not run down easily.
The oscillation circuit shown in FIG. 4 is a conventional oscillation circuit where a decrease of power consumption is attempted. This oscillation circuit 101 is comprised of a piezoelectric oscillator 104 for generating voltage that oscillates at a natural frequency, an inverter 111 for amplifying this oscillation voltage, a feedback resistance 105, load capacitors 106 and 107, and a constant voltage source 115 that is connected to a power supply line 110 from a battery 103 for supplying constant voltage to the power supply of the inverter 111. To the internal circuit 113 for counting time, of which power supply is connected to the power supply line 110 of the oscillation circuit 101, output of the inverter 111 of the oscillation circuit 101 is input. In this oscillation circuit 101, the constant voltage that is output by the constant voltage source 115 is set to the minimum voltage at which the inverter 111 can operate, so as to decrease the power consumption of the oscillation circuit 101.
The oscillation circuit shown in FIG. 5 is another conventional oscillation circuit where a decrease of power consumption is attempted. This oscillation circuit 102 comprises a constant current source 114 instead of the constant voltage source 115 of the above-mentioned oscillation circuit 101. In this oscillation circuit 102, a decrease of power consumption is attempted by limiting the current by the constant current source 114 when short-circuit current flows through the inverter 111.